Gtk3 theme inspired by Material design

Paper is a new material design inspired GTK theme, which is currently in beta. "Its design is mostly flat with a minimal use of shadows for depth", mentions its website, and it was developed primarily for the GNOME (Shell) and other desktops that make use of header bars.
Ideally, you'll need a complete GNOME 3.14 / 3.12 desktop to see what Paper is all about, since the theme uses custom colors for header bars / client-side decoration apps.